If you don't have hours to spend choosing and maintaining an asset allocation for your retirement account, consider a lifecycle fund, sometimes called a target-date fund. A lifecycle fund is an all-in-one investment option that offers you, in a single fund, a diversified portfolio with an asset allocation geared to the year in which you expect to retire.
A lifecycle fund's asset allocation reflects what its investment managers have determined is the optimal risk and return profile for a given time horizon ending with the fund's target year. You simply choose a fund with a target year that lines up with when you expect to retire. The target year is identified in the fund's name, making it easier for you to find a fund that's appropriate for you. For example, if you're 35 today and expect to retire in about 30 years, or in 2047, you might look for a fund with 2047 in its name.